Conventionally, a glow pug used for, for example, assisting startup of a diesel engine includes a tubular metallic shell, a rod-shaped center shaft, a heater including a heat-generating element which generates heat when energized, an insulating member, an outer sleeve, a connection terminal, etc. In view of the required performance of diesel engines and costs, there have been used a metal glow pug which includes a sheathed heater having a metal sheath, and a ceramic glow plug which includes a ceramic heater.
Incidentally, such a ceramic glow plug generally has the following structure. Namely, a center shaft is disposed inside a metallic shell such that its one end projects toward the rear end side, and a ceramic heater (hereinafter, may be simply referred to as a “heater”) is provided on the forward end side of the center shaft. Also, an outer sleeve formed of metal is joined to a forward end portion of the metallic shell, and the heater is held by the outer sleeve. Meanwhile, on the rear end side of the metallic shell, an insulating member is inserted between the center shaft and the metallic shell. On the rear end side of the insulating member, a connection terminal is fixed to the center shaft in a state in which the connection terminal pushes the insulating member toward the forward end side. Preferably, a method of press-fitting the heater into the outer sleeve is used to hold the heater. At that time, there may be used a method of applying a lubricant to the heater so as to facilitate the press fitting, and removing the lubricant by heating after completion of the press fitting. Thus, a radially inward force from the outer sleeve acts on the heater, whereby the heater is firmly constricted and held.
The above-described ceramic heater is formed by embedding and holding a heat-generating element formed of an electrically conductive ceramic in a substrate formed of an electrically insulating ceramic. In this case, electrode taking-out portions of negative and positive poles used for applying a voltage to the heat-generating element are provided at the rear end side of the ceramic heater. One electrode taking-out portion is electrically connected to the metallic shell, and the other electrode taking-out portion is electrically connected to the center shaft (see, for example, Patent Document 1). These electrical connections are realized by the above-mentioned press fitting. The two electrode taking-out portions are connected to opposite end portions of the heat-generating element through a pair of rod-shaped lead portions. Similar to the heat-generating element, the two electrode taking-out portions and the pair of lead portions are formed from an electrically conductive ceramic (see, for example, Patent Document 2). Hereinafter, the electrode taking-out portions, the lead portions, and the heat-generating element may be collectively referred to as a “resistor.”